1. Field of the Invention
The invention relates to a method for setting a termination voltage for an input circuit of an integrated circuit. The invention furthermore relates to an input circuit for an integrated circuit which provides an optimized termination voltage.
2. Description of the Related Art
Particularly in the case of data connections between DRAM memory modules and the associated memory controller, the data transfer takes place at a high frequency. The reception of the signals is sensitive to systematic signal distortions, such as, e.g., asymmetries between the low level and the high level of the received signal.
The data transfer between the memory controller and the DRAM memory module is performed by a driver circuit in the transmission part and an input circuit in the reception part. The driver circuit pulls the potential of the corresponding data line to a high or to a low potential with respect to the impedance of the data line and the input impedance of the input circuit. The range in which the high level or the low level of the transferred signal lies is determined by the component parameters of the driver circuit and also by the electrical resistances of the data line and input circuit. The center potential about which the signal to be transferred varies is prescribed by the input circuit in the form of a termination voltage. The high and low levels of the signal are more or less symmetrical with regard to the termination voltage depending on the intrinsic driver power of a pull-up path and a pull-down path of the driver circuit and/or depending on the noise of the respective supply potential which is connected to the data line via the pull-up or pull-down path.
The received signal present at an input terminal of the input circuit is usually compared with an externally prescribed reference voltage assess the signal value of the received signal. The termination voltage is generated in the input circuit independently of the externally prescribed reference voltage. Consequently, the termination voltage and the reference voltage may be different depending on process-dictated component deviations. If termination voltage and reference voltage are not identical, then the received signal may be asymmetrical with respect to the reference voltage. However, the reference voltage is provided in identical fashion for all the input circuits of the integrated circuit, with the result that deviations of individual termination voltages cannot be compensated for by changing the reference voltage.